Project Summary/Abstract Adolescent alcohol use/misuse remains a substantial public health concern in the United States, given its acceleration across this developmental period and resulting negative consequences (e.g., drunk driving, academic/occupational impairment, morbidity and mortality). While alcohol use, on average, increases across adolescence into emerging adulthood, research has also identified subgroups of adolescents whose drinking follows distinct patterns, or trajectories, over time. These distinct trajectories have differing implications for long-term health outcomes (e.g., later alcohol and substance use disorders), necessitating research on the multifaceted and developmentally-specific contributors to trajectory membership. Developmental theories posit multiple pathways to alcohol use that are influenced by diverse, intersecting risks whose salience change across developmental stages. Given the salience of peer influences in adolescence, high-risk peer environments may promote membership in more problematic adolescent drinking trajectories. Further, the association between peer environments and alcohol outcomes may be modulated by underlying genetic propensities through gene-environment (GE) interactions. Although peer socialization in adolescent alcohol use, whereby adolescents match their peers' drinking and risky behavior, has been well-documented, research on GE interaction effects on membership in alcohol use trajectories has been limited. Genetic differences in peer environmental effects may be conferred by both candidate single nucleotide polymorphisms (SNPs) in alcohol metabolism genes as well as additional genetic variants associated with alcohol use/misuse in genome-wide association studies (GWAS). Specifically, alcohol dehydrogenase gene variants may attenuate peer socialization, such that adolescents carrying protective variants are less likely to follow problematic drinking trajectories than noncarriers. This project will perform secondary data analyses on a longitudinal, population-based cohort of over 15,000 adolescents to test the independent and interactive associations of genetic (i.e., polygenic risk scores involving candidate alcohol metabolism genes and SNPs identified in an exploratory GWAS) and peer-based (i.e., peer alcohol/substance use and disruptive behavior) contributors to membership in adolescent drinking trajectories. Findings would advance our understanding of the multifaceted contributors to the etiology and acceleration of alcohol use/misuse across the critical adolescent period.